Add missing return statement

The return statement was missed when vrouter_api.py was modified to use REST interface instead of Thrift.

Closes-Bug: #1664819
(cherry picked from commit 392d0be)

Change-Id: I4c1adb4eafc09eb410f3713e2957c27553b015f9

src/vnsw/contrail-vrouter-api/contrail_vrouter_api/vrouter_api.py

@@ -90,6 +90,9 @@ def add_port(self, vm_uuid_str, vif_uuid_str, interface_name, mac_address,
                 self._ports[vif_uuid_str] = cmd
 
             self._resynchronize()
+            if (ret_code != 0):
+                return False
+            return True
 
         finally:
             if self._semaphore: